Microwave and terahertz radiation can be generated either by a solid-state device based on semiconductors, a vacuum electron device (VED) based on the interaction between an electron beam and an electromagnetic circuit or by photonics based sources such as lasers. In the millimeter wave (30-300 GHz) regime VED based sources are dominant because of their high power, wide bandwidth, efficiency and robustness when compared to solid-state and photonics based devices. The aforementioned advantages of VEDs are also valid in the terahertz (300 GHz-3 THz) regime especially, between 300 GHz-1 THz. In this part of the electromagnetic spectrum VED based devices can provide 3 to 4 orders of magnitude higher peak and average power than solid-state devices and are significantly compact and approximately an order of magnitude more efficient than photonics based devices.
The applications of millimeter waves in the area of defense, communication, industrial processing and scientific applications are very well known. Recently, the advent of various kinds of terahertz sources has sparked considerable interest in applications of terahertz sources in the above applications and notably in the area of medical imaging, therapy and other biotechnological applications.
The art of generating microwave and millimeter wave frequencies using an electron beam coupled to an electromagnetic structure (circuit) is well known. A VED is a vacuum tube that has the following main components in a vacuum envelope. An electron gun which generates the electron beam either by thermionic emission from a metal such as tungsten impregnated with other metals and compounds or by field emission. The electron beam, which is used as a means to transform the DC energy provided by a high voltage power supply to microwave or terahertz radiation in the circuit. The electron beam gains kinetic energy from the high voltage power supply and it transfers this kinetic energy to the microwave/terahertz waves in the interaction structure. The magnetic system is used to focus and transport the electron beam from the electron gun to the interaction region where microwaves are generated or amplified. In some classes of VEDs like gyrotrons, the magnetic system also plays a role in the interaction between the electron beam and the circuit. In almost all VEDs, strong magnetic fields are necessary to focus the beam by overcoming the space forces inherent in the electron beam. In gyrotrons the required magnetic field is directly proportional to the cyclotron frequency of the electron beam, which is critical for its interaction with the electromagnetic structure. The gyrotron can be operated either at the fundamental mode or higher harmonics of the cyclotron frequency. Higher harmonics have the advantage that a lower magnetic field for operation is required, depending on the harmonic number. The conversion of the kinetic energy in the electron beam to microwave or terahertz waves takes place in the interaction structure. The resonant frequency of the structure is tuned to interact with the electron beam at a specific frequency or a range of frequencies. The interaction structure is also referred to as cavity or a resonator. The interaction structure operates in either the fundamental or a higher order Transverse Electric (TE) or a Transverse Magnetic (TM) mode. Such a mode cannot be efficiently extracted out of the microwave tube or transported over long distances at low loss. Hence, an internal mode converter is often employed to transform the operating mode to a different mode, typically, a free space Gaussian-like mode. The internal mode converter can be designed on the basis of standard waveguide design techniques or quasioptical methods. The body of the VED is held at high vacuum. The window allows the microwaves or terahertz radiation generated in the VED to be extracted out of the tube. The spent electron beam after giving some of its energy to the microwave or terahertz fields continues traveling to the anode and is typically collected on a copper structure called the collector. The collector is either air- or water-cooled to absorb the remaining energy of the spent beam, which is dissipated as heat in the collector. For devices based on fast-wave interaction between the electron beam and the electromagnetic circuit such as gyrotrons, at least 0.036 Tesla/GHz of magnetic field is required for fundamental electron cyclotron operation. At microwave and terahertz frequencies several Tesla (T) of magnetic field is necessary. This magnetic field is typically provided by a superconducting magnet which is large and expensive.
Terahertz radiation is used in many fields of magnetic resonance spectroscopy for example in electron paramagnetic/spin resonance (EPR/ESR) spectroscopy. In EPR Spectroscopy, paramagnetic systems are studied by their microwave absorption in a strong magnetic field. At high magnetic fields solid-state sources deliver very little power and high-power microwave sources are desired. Dynamic Nuclear Polarization (DNP) is a technique that utilizes the large Boltzmann polarization of the electron spin reservoir to provide a boost in NMR signal intensities by several orders of magnitude, thus dramatically increasing the data acquisition rate in a NMR experiment. This makes DNP a valuable method to overcome the intrinsic low sensitivity of liquid- and solid-state NMR experiments and the method is of significant interest in applications ranging from particle physics to structural biology. In clinical imaging, this method can be especially valuable in improving contrast and resolution in Magnetic Resonance Imaging (MRI). Several other techniques are available to increase the signal intensity in an NMR experiment, such as para-hydrogen induced polarization (PHIP), polarization of noble gases such as Helium, Xenon or Krypton and optically pumped nuclear polarization of semiconductors. However, especially in the area of structure determination of bio-macromolecules or bio-solids, microwave driven DNP-enhanced NMR spectroscopy seems to be the most versatile method.
DNP is not a new scientific area but is currently experiencing a renaissance with the advent of high-power microwave and terahertz sources. The first DNP experiments were performed in the early 1950s at low magnetic fields but until recently, the technique was of limited applicability because of the lack of high-frequency, high-power terahertz sources. In a DNP experiment, the large electron polarization of a polarizing agent is transferred to surrounding nuclei (typically protons, 1H) by terahertz (microwave) irradiation on-resonance with an EPR transition. The electron spin system required for DNP can either be an endogenous or exogenous paramagnetic system. To date most polarizing agents for high-field DNP experiments are based on TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) moieties, which employ the cross-effect (CE) as the DNP mechanism. The CE mechanisms can be exploited, if the homogenous linewidth (δ) and the inhomogeneous breadth (Δ) of the EPR spectrum of the paramagnetic polarizing agent, is larger compared to nuclear Larmor frequency (ω01). The underlying mechanism is a two-step process involving two electrons with Larmor frequencies ω0S1 and ω0S2 separated by the ω01 (matching condition). The DNP-enhanced nuclear polarization then disperses throughout the bulk via spin diffusion.
Four components are essential to perform DNP-enhanced NMR spectroscopy: A high-power, high-frequency terahertz source; a low-loss transmission line; a NMR probe, that allows simultaneous terahertz and radio-frequency irradiation of the sample and a polarizing agent as the source of the high thermal electron polarization (briefly described above).
The efficiency of a DNP experiment depends strongly on the magnetic field (B1) at the sample, which is induced by the terahertz irradiation. The strength of this field, B1 is proportional to √{square root over (PQ)} with P, the microwave power and Q, the quality factor of the cavity. Typically, DNP experiments are performed without a resonant structure (cavity) thus, the quality factor Q is small (<5). To create sufficiently strong B1 fields high-power terahertz sources such as gyrotrons are required.
In current DNP setups, the microwave/terahertz source is completely independent from the magnetic resonance system and requires an additional superconducting magnet or a conventional high-field electromagnet or a permanent magnet system. The microwave/terahertz radiation is delivered to the sample using a transmission line. Often the main magnet and the superconducting gyrotron magnet have to be separated sufficiently to avoid any interference. Throughout this background and the rest of the disclosure magnetic resonance system is meant to include all such devices e.g. nuclear magnetic resonance (NMR), electron paramagnetic, resonance (EPR), magnetic resonance imaging (MRI) . . . .
Low-loss microwave/terahertz transmission lines are required to efficiently deliver the terahertz radiation from the source (e.g. gyrotron) to the sample. To ensure minimal transmission losses a corrugated metallic waveguide can be used. Operation in the HE11 mode of a corrugated metallic waveguide results in negligible ohmic losses thus enabling high efficiency transmission over long distances. A quasi-HE11 mode can be generated inside the VED using an internal mode converter and the radiation can be directly coupled to the transmission line. Furthermore, in a solid-state DNP experiment the HE11 mode can be used to directly illuminate the sample. For DNP-enhanced solid-state NMR experiments a low-temperature, magic-angle-spinning (MAS) NMR probe is required. Cryogenic temperatures are achieved by either using cold nitrogen as the bearing and turbine gas or by using a separated variable-temperature line for sample cooling. Temperatures down to 85 K can be reached using this approach. If lower temperatures are necessary cold He gas, as blow-off gas directly from a liquid He Dewar can be used for cooling. To maintain the possibility to tune the radiofrequency (RF) circuit even at low temperatures a transmission line circuit is typically employed. Here all variable tuning elements are located outside the probe at room temperature. The terahertz radiation can be introduced either along the rotor axis or perpendicular. In the later case, the radiation enters the sample through diffraction between the turns of the RF coil. In such setup, due to the sample rotation a majority of the sample is uniformly exposed to the terahertz radiation, thus increasing the efficiency of the DNP process. To change samples without removing the probe from the magnet, a sample eject system can be added to the probe.
There are a number of shortcomings to the conventional approaches. Both magnets, used to perform the magnetic resonance experiments (main magnet) and the magnet for the microwave generator are large and expensive and each requires cryogenic cooling. A microwave waveguide is necessary to deliver the terahertz energy from the microwave generator to the sample area located in the main magnet and it must be of sufficient length to accommodate adequate distance between the main magnet and the microwave generator magnet to prevent field interference. Substantial beam power in the gyrotron (e.g. >2 kW) is required and the conversion is relatively inefficient especially in the second harmonic operation: the microwave output is only 10-20 watts.